In general, a construction machine such as a hydraulic excavator comprises an engine, at least one variable displacement hydraulic pump driven by the engine, a plurality of hydraulic actuators driven by a hydraulic fluid delivered from the hydraulic pump, a plurality of flow control valves for controlling respective flow rates of the hydraulic fluid supplied from the hydraulic pump to the plurality of hydraulic actuators, and a plurality of control lever devices serving as operating means to operate the plurality of flow control valves. Also, a diesel engine is employed as the engine for driving the hydraulic pump. The diesel engine is equipped with a fuel injector, called a governor, to control an amount of fuel injected, thereby controlling a revolution speed of the engine.
In such a diesel engine equipped with a fuel injector, when a control lever of the control lever device is quickly manipulated for shift of the flow control valve, an input torque (load) of the hydraulic pump is abruptly increased and the engine revolution speed abruptly drops. This abrupt drop of the engine revolution speed leads to problems of not only deteriorating fuel consumption and exhaust gas, but also causing noises.
Techniques for suppressing such a drop of the engine revolution speed are disclosed in, for example, JP,A 2000-154803 and JP,A 2001-173605.
With the technique disclosed in JP,A 2000-154803, the load state of a hydraulic pump is detected, and when it is detected that a load is applied to the hydraulic pump, a limit value for the input torque of the hydraulic pump is reduced to perform torque decrease control. As a result, the absorption torque of the hydraulic pump (i.e., the engine load) is reduced so as to suppress the drop of the engine revolution speed.
With the technique disclosed in JP,A 2001-173605, the operating speed of a control lever is detected, and when the operating speed exceeds a predetermined value, fuel is supplied in an increased amount to an engine in response to a command signal from a controller. As a result, the engine output is increased so as to suppress the drop of the engine revolution speed.